In the course of a life that nearly spanned the 16th century, that glorious age of exploration, a Flemish peasant's son, Gerard Mercator, helped shape the modern perception of the planet while seldom venturing beyond the confines of a corner of northwestern Europe. Crane (Clear Waters Rising), a British geographer and adventurer, makes much of Mercator's long life and uses this longevity as an organizing theme of the biography: "surviving for twice as long as many of his contemporaries, he was able to mature through two consecutive life spans." In the first half of his life, the comparatively impetuous Mercator, struggling with his ideals, was imprisoned under the inquisition. In the second, with his passions more focused, he conceived and drew the first modern map using a "projection" that solved certain navigational problems; eventually, he created the first unified compilation of maps of the world, called an atlas. The raw material here is rich: there's the story of a poor boy makes good, explorations into civil and martial turmoil, and the excitement of new discoveries. While Crane sometimes loses track of the main story amid the minutiae of shipping manifests, he does demonstrate a real talent for incorporating letters and documents from diverse sources into very readable prose, as well as teasing Mercator's personality out of sometimes scant or tangential sources. (Jan.)